Hi Christo,
welcome to the site
Just thermostats? What would they be controlling if not the heater element and how do you want to use them? I am not sure a sling nursery is even necessary, as long as the slings are in a climate comfortable to you and not colder than say 15deg celcius (I assume you are in the south african climate?), they should be fine and you will not need to control anything, unless of course you want to keep them warm, active and feeding well through winter.
Have you got yourself a copy of the Tarantula Keepers guide yet?
Good luck.
HS